I am running simulations of a ring shear tester and I use the fix mesh/surface/stress/servo command.
I am not completely sure about the way to calculate the tangential shear stress from the values of forces and torques I obtain from LIGGGHTS. As result, I have a global vector with 9 components for access by various output commands. The first 3 components are equal to the total force on the mesh, the next 3 components store the total torque on the mesh.
The shear stress would then be something like the total force (so sqrt (Fx^2+Fy^2+Fz^2))/(surface of top lid), or rather the total torque/(surface of the lid* moment arm)?
Basically, how the values of tangential shear stress reported here are calculated: https://www.cfdem.com/stress-controlled-wall-servo-wall
Thank you in advance for your help,